In aircraft design, many actuators are required to move certain movable elements of the craft relative to other reaction elements. One example of such a use is in the actuation of flaps on a conventional fixed wing aircraft. As in any aircraft environment, the actuator would ideally be extremely reliable, lightweight, compact, and require a minimum energy input for actuation, among other requirements.
The ball screw actuator is commonly employed in aircraft applications. Simply put, a ball screw actuator includes a nut with internal threads and a screw with external threads. A plurality of spherical balls are captured within the threads of the nut and engage the threads on the screw. Rotation of the nut about its center axis while resisting similar rotation of the screw will cause the screw to move axially through the nut.
Clearly, the nut could be mounted on a reaction element and the screw on a moving element in an aircraft to form an actuator. While the ball screw actuator is much less resistant to jamming than a simple threaded engagement between a nut and a screw, jamming can occur. Further, balls may be lost from the nut, reducing or eliminated the ability to translate the screw axially. Further, the screw may be the only fixed connection between the moving element and the reaction element and, should the screw crack apart, catastrophic results could ensue.
In recent years, an aircraft has been under development which is commonly referred to as a tilt rotor aircraft. In this aircraft, a rotor or propeller, and its associated power plant, is mounted on a pylon which can pivot on the aircraft between a conventional flight mode and a helicopter mode. In the conventional flight mode, the propeller rotates in a vertical plane to drive the aircraft forward as in a conventional prop driven aircraft. However, the pylon and propeller can then be converted or pivoted to position the propeller in essentially a horizontal plane, where it can act as a helicopter rotor and the aircraft operated as a helicopter for vertical takeoff and landing. In such an environment, a highly reliable and efficient actuator is necessary for the proper operation of the pylon in converting between the aircraft mode and the helicopter mode.